In conventional electrostatic propulsion engines of the above mentioned general type, atoms of a propellant gas expelled from a supply container or tank are first ionized to form positively charged propellant ions, and then these ions are accelerated in an electrostatic high voltage field to form a high-energy beam or jet of the ions which in turn provides a propulsive thrust. In order to maintain a constant drive thrust output, in this context, it is absolutely necessary to provide suitable measures for neutralizing the positively charged propellant ion beam or jet emitted from the engine. Preferably, a gas discharge arrangement serves as a neutralizer, in that it is used as an electron source providing electrons that neutralize the positively charged ions.
Along these lines, it is already known to provide a cathode tube having a gas flowing therethrough and an anode that is referred to as a keeper electrode, and to generate a hollow cathode gas discharge therebetween. Then, free electrons are extracted from this hollow cathode gas discharge and are then coupled into the beam or jet of the emitted propellant ions in a suitable manner so as to neutralize the positive ions.
In an arrangement of the above described type, in order to initiate the gas discharge between the anode and the cathode it is necessary to heat up the cathode relatively strongly, so that the emitted electrons have a tendency to ionize the gas flowing through the cathode tube, due to the applied anode voltage, and thereby initiate the discharge process. Such a cathode is generally made of a material having a high electron emission capacity, such as impregnated tungsten for example, and it is typically necessary to heat such a cathode to a temperature of approximately 1200.degree. C. Not only does this heating require a considerable expenditure of energy, but the required high cathode temperature leads to high loads and demands being placed on the material, which in turn leads to accelerated and early material fatigue. Moreover, it is necessary to provide a relatively complex and costly arrangement of the entire apparatus, to ensure that it will be thermally and mechanically stable under the high temperature loading conditions and the resulting great temperature gradient and variation. Also, this known apparatus requires a high throughput or flow rate of the gas in order to initiate and maintain the ignition.